1. Field of the Invention
The present invention generally relates to methods and systems for selecting one or more modes of an inspection subsystem or system for inspection of a specimen.
2. Description of the Related Art
The following description and examples are not admitted to be prior art by virtue of their inclusion in this section.
Inspection processes are used at various steps during a semiconductor manufacturing process to detect defects on wafers to promote higher yield in the manufacturing process and thus higher profits. Inspection has always been an important part of fabricating semiconductor devices. However, as the dimensions of semiconductor devices decrease, inspection becomes even more important to the successful manufacture of acceptable semiconductor devices because smaller defects can cause the devices to fail.
Many inspection tools have adjustable parameters for many of the output (e.g., image) generation elements of the tools. In this manner, the parameters for one or more elements (such as energy source(s), polarizer(s), lens(es), detector(s), and the like) can be altered depending on the type of specimen being inspected and the characteristics of the defects of interest (DOIs) on the specimen. For example, different types of specimens may have dramatically different characteristics, which can cause the same tool with the same parameters to image the specimens in extremely different ways. In addition, since different types of DOIs can have dramatically different characteristics, inspection system parameters that are suitable for detection of one type of DOI may not be suitable for detection of another type of DOI. Furthermore, different types of specimens can have different noise sources, which can interfere with detection of DOIs on the specimens in different ways.
The development of inspection tools with adjustable parameters has also led to the increasing use of inspection processes that involve scanning the specimen with more than one combination of parameter values (otherwise referred to as “modes”) such that different defect types can be detected with different modes. For example, one mode may have a greater sensitivity for detecting one type of defect while another mode may have a greater sensitivity for detecting another type of defect. Therefore, using both modes, an inspection system may be able to detect both types of defects with acceptable sensitivity.
Several currently used methods are available for optical mode selection (OMS), which may be performed to find the best mode for defect inspection. Currently, OMS methods and systems are limited in their ability to assess the best optics mode with design-based care areas (CAs) and design-based accuracy since every mode needs to have design-based alignment and runtime context mapping performed, which is time consuming, risky, and could potentially fail.
In particular, one disadvantage of currently used methods is that design-based accuracy for CAs can only be used for the base test mode in OMS. All other modes cannot benefit from the CA placement accuracy. Such CA placement accuracy is, however, especially critical when defects are located at the edge of a memory device area (such as a static random access memory (SRAM) area) where only use of design-based placement accuracy can guarantee a cell-to-cell based inspection can be performed whereas non-design-based CAs would require a die-to-die based inspection, which is typically lower in sensitivity. In addition, the design cannot be overlaid onto the patch images generated in OMS in the current OMS setup. However, such overlay of the design on the images is very often critical to make sure that the right defect location can be identified. Furthermore, if substantially small CAs (e.g., 7 pixels by 7 pixels) have to be used for nuisance suppression in an inspection, then there is no way in the current OMS setup to guarantee that an area that defines where the defect of interest (DOI) is located (called a “signal box”) remains stable at that position for all of the modes selected for use in an OMS process. Slight misplacements of the signal box can lead to different signal-to-noise (S/N) values, which could lead to the wrong best mode.
Accordingly, it would be advantageous to develop systems and methods for selecting one or more modes of an inspection subsystem or system for inspection of a specimen that do not have one or more of the disadvantages described above.